Recently, as materials used for a rotor blade of various turbines, TiAl-based alloys, being lightweight and having excellent heat resistance, have attracted attention. Particularly, in the case of a large rotatable rotor blade, as the constituent member of the rotor blade become lighter, the centrifugal stress becomes smaller, which enables improvement in the maximum engine speed, an increase in area of the rotor blade, and a reduction in the load stress applied to a disk portion of the rotor blade and is very beneficial to the increase in efficiency of the entire apparatus.
This TiAl-based alloy is an alloy composed mainly of TiAl or Ti3Al, which is an intermetallic compound having excellent high-temperature strength, and the alloy is excellent in heat resistance as described above. The TiAl alloy, which is a lightweight heat resistance alloy, is used as a casting material and a forged material.
The casting material has a perfect lamellar structure laminated with a α2-phase and a γ-phase which are excellent in high-temperature strength, but there is a problem that room-temperature ductility is deficient because forgeability is poor and a crystal grain is coarsened. Therefore, for example, a technique is proposed in Patent Literatures 1 and 2, in which a TiAl-based alloy material as a hot forging material having a predetermined composition is held in an equilibrium temperature range of (α+β)-phase and is then subjected to plastic working, thereby eliminating casting defects and fining a structure by a synergistic effect of working distortion and phase transformation. Moreover, thereafter, the hot-forged TiAl-based alloy material is held in an equilibrium temperature range of (α+β)-phase, (α+β+γ)-phase, or (β+γ)-phase, an area fraction of lamella grain and β-phase or a grain size of the lamella grain is controlled, and thus the TiAl-based alloy having excellent machinability and high-temperature strength can be produced. As a hot working method other than the hot forging, for example, extrusion or rolling-type forging can be used.